#define _CRT_SECURE_NO_WARNINGS
#include <iostream>

using namespace std;

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode* left, TreeNode* right) : val(x), left(left), right(right) {}
};

class Solution {
public:
    long prev = LONG_MIN;
    bool isValidBST(TreeNode* root) {
        return dfs(root);
    }

    bool dfs(TreeNode* root)
    {
        if (root == nullptr)
            return true;

        bool left = dfs(root->left);
        if (left == false)
            return false;

        bool cur = root->val > prev ? true : false;
        prev = root->val;
        if (cur == false)
            return false;

        bool right = dfs(root->right);

        return left && right && cur;
    }
};

int main()
{
	return 0;
}